Telescope Basics - Celestron G-3 Instruction Manual

Focusing
Focusing the G-5 telescope:
The G-5 telescope has a focusing knob located on the rear cell of the optical tube to the right of the star
diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the knob will not turn, the
primary mirror has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite
direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer
object and counterclockwise for a more distant object. For astronomical viewing, out-of-focus star images
are very diffuse making them difficult, if not impossible, to see. For best results, your first astronomical
target should be a bright object (like the Moon) so that the image is visible even when out of focus.
Focusing the G-3 telescope:
To focus the G-3 telescope turn the focuser ring (see figure 1) on the optical tube until the image is sharp.
For objects closer than the one you are currently observing, turn the focusing ring clockwise and for objects
further away, turn counterclockwise. The G-3 has a tremendous amount of focus travel to allow for critical,
sharp focusing. For astronomical viewing, out-of-focus star images are very diffuse making them difficult, if
not impossible, to see. For best results, your first astronomical target should be a bright object (like the
Moon) so that the image is visible even when out of focus.
Calculating Magnification
The magnification (or power) of a telescope is variable depending upon the focal length of the eyepiece
being used along with the focal length of the telescope.
To calculate the magnification we use the following formula:
where FL(telescope) is the focal length of the telescope and FL(eyepiece) is the focal length of the
eyepiece. For example, if the focal length of the G-3 telescope is 1000mm and you use a 25mm
eyepiece your magnification is 1000/25 = 40x.
There are numerous optional eyepieces that are available to expand your range of magnification. There
are various types of eyepieces of varying quality to choose from.
The Celestial - Coordinate System
The celestial-coordinate system is an imaginary projection of the Earth's geographical coordinate system
onto the celestial sphere which seems to turn overhead at night. This celestial grid is complete with equator,
latitudes, longitudes and poles.
The Earth is in constant motion as it rotates on its axis. Actually the celestial-coordinate system is being
displaced very slowly with respect to the stars. This is called precession and is caused by gravitational
influences from the Sun, Moon and other celestial bodies.

Telescope Basics

FL telescope (
Magnification
FL eyepiece (
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